1. Field of the Invention
The present invention relates to an image forming apparatus adopting an electrophotographic method such as a copying machine and a laser beam printer. More specifically, the present invention relates to an image forming apparatus capable of forming a multi-color image, comprising a plurality of image forming sections.
2. Description of the Prior Art
In a color image forming apparatus which is an image forming apparatus comprising a plurality of image forming sections, color images have been hitherto formed by superimposing various color images on a transfer member (recording material) in a sheet form. For example, with a digital color copying machine, a document image color-separated and input by a scanner is then subjected to a predetermined image processing. Thereafter, an image is formed for each color by a plurality of image forming sections provided for each color, and these images are superimposed on a recording paper to obtain one color image.
With these digital color copying machines, images of respective colors are faithfully reproduced and superimposed on a recording paper with high accuracy, hence a high grade image can be reproduced with high fidelity without impairing the image expression which the document image has.
Therefore, process control for controlling the image forming conditions in image forming sections, and resist adjustment for controlling the image forming position so that each color image is superimposed on the recording paper with high accuracy have been recently executed, so that color reproduction can be performed with high fidelity in the image forming sections for each color in order to output an image closer to the document image.
The technology relating to these process control and resist adjustment has been disclosed, for example, in Japanese Patent Application Laid-open Hei 5 No. 119578 and Hei 5 No. 100578, and Japanese Registered Patent Publication No. 2642351.
In Japanese Patent Application Laid-open Hei 5 No. 119578 and No. 100578, there is described an image forming apparatus which detects the toner density of a test image transferred for each image forming section immediately after the transfer to thereby control each image forming process.
Particularly, in Japanese Patent Application Laid-open Hei 5 No. 119578, it is described that the image density is properly controlled according to a density detection signal. In Japanese Patent Application Laid-open Hei 5 No. 100578, it is described that the transfer current of transfer means is controlled according to a density detection signal.
On the other hand, in Japanese Registered Patent Publication No. 2642351, it is described that test images formed in respective image forming sections are respectively transferred onto a transfer carrier belt, and each test image is read by a single sensor provided on the downstream side in the direction carrying a transfer medium, to determine the positional relationship of each test image, and to control the image forming position of each image forming section.
To perform the above described process control and resist adjustment with high accuracy, however, it is necessary to accurately read the density and forming position of each test image, which is formed by each image forming section and becomes a basis of the control and adjustment. That is to say, if read of the test image is incorrect, highly accurate control and adjustment cannot be performed.
According to the technique described in the above described Japanese Patent Application Laid-open Hei 5 No. 119578 and No. 100578, a sensor is provided for each image forming section so that a test image is read for each image forming section. Hence, it is useful from a standpoint that since an image formed in each image forming section and transferred onto the transfer carrier belt is read immediately after the transfer, the test image can be read with high accuracy.
If a plurality of sensors are used, however, there is a problem that the image is affected by the difference of detection results between respective sensors. Particularly, in the resist adjustment, the positional detection of each test image may be not correct due to the difference of the attached position between a plurality of sensors, hence the accuracy of the resist adjustment deteriorates. Moreover, since expensive sensors are arranged in plural numbers, cost increase cannot be avoided. Furthermore, there is another problem in that space and wiring for arranging a plurality of sensors and space for a circuit portion are required.
On the contrary, according to the technique described in Japanese Registered Patent Publication No. 2642351, detection is performed by a single sensor provided on the downstream side in the direction carrying a transfer medium, enabling to prevent the above described cost increase, difference of detection results between a plurality of sensors, and problems of additional space, which makes is useful.
However, it has a construction that a test image formed in each image forming section is sequentially transferred onto the transfer carrier belt. Therefore, it may cause such a situation that a test image formed in an image forming section on the upstream side and transferred onto the transfer carrier belt is re-transferred to a photosensitive material in an image forming section on the downstream side, when passing through the image forming section, resulting in a state different from that of at the time of transfer.
Below is a description of the mechanism and principle which cause the above described re-transfer. FIG. 1 shows a construction of one image forming section, which comprises, around a photosensitive drum 222, a charging process by means of an electric charger 223 for uniformly charging the photosensitive material surface to a predetermined electric potential; an image exposure recording process for writing an image; a development process by means of a developing device 224 for reproducing an image by adding a developer to a portion where the image has been written; a transfer process by means of a transfer device 225 for transferring the image reproduced on the photosensitive material 222 onto a transfer medium (a transfer carrier belt 216); a cleaning process by means of a cleaner 226 for enabling the next image forming by removing the developer remaining on the photosensitive material 222; and a discharging process by means of a discharger for removing the residual potential on the photosensitive material surface and enabling the stabilized next image forming. By repeating these processes, images are recorded.
In the conventional digital color copying machine, when a test image is formed on the transfer carrier belt 216, and the position of the test image is read to be resist adjusted, transfer voltage of +1.2 kV is always applied on the transfer means 225 even when the image is transferred from the photosensitive drum 222 and when the test image transferred on the transfer carrier belt 216 passes therethrough.
FIG. 2 shows the transition of the potential state on the photosensitive material 222 of the image forming section shown in FIG. 1. Next is a description of the transition by dividing it into (1) charging process, (2) exposure process, (3) development process, and (4) transfer process. (1) The surface of the photosensitive material 222 is uniformly charged to -500 V by the electric charger 223. (2) The potential of the photosensitive material where the image is written (image portion) drops to several tens V, causing the potential difference between the image portion and a non-image portion (the surface potential of the photosensitive material uniformly charged in the charging process drops gradually). (3)Developing bias of -200 V is applied to a developing roller to attach a negatively charged toner to the image portion on the photosensitive material 222 by stirring the toner and the carrier, so that the toner is attached only to the image portion which is on the 0 V side from -200 V (hatched area in FIG. 2). (4) Transfer bias of +1.2 kV is applied to the transfer device 225 to electrically draw the toner, in order to transfer the toner image attached on the photosensitive material 222 onto the transfer medium (transfer carrier belt 216).
Here, since voltage of +1.2 V is always applied to the transfer device, the photosensitive material surface is positively charged due to the high transfer bias. Therefore, the toner of the test image once transferred (the toner is negatively charged), or the toner of the test image transferred in the image forming section on the upstream side on the transfer medium 216 is drawn toward the photosensitive material in a portion after the transfer section of the photosensitive material 222 (a position in the vicinity where the photosensitive material 222 parts from the transfer carrier belt 216). In particular, with regard to the test image formed in the other image forming sections, the retaining force of the toner drops while being carried, hence those test images are easily drawn toward the photosensitive material 222.
The above is the mechanism for re-transfer of the image. If such re-transfer is caused in the test image for performing the process control and the resist adjustment, edges of the test image are blurred, and the position (or the pattern interval) cannot be detected accurately. Moreover, if the toner density becomes low, accurate density adjustment cannot be performed.
As a result, with the conventional construction, it cannot be said that detection of the test image is always correct, hence the control based on the detection is neither correct. Thus, there is a problem that a color image faithful to the document image cannot be reproduced.